Complementary Protective MeasuresControl FunctionsGeneralRobotics

Why you should stop using the term ‘Deadman’

The Deadman Control

Do you use the phrase ‘dead­man’ or ‘dead­man switch’ when talk­ing about safety-related con­trols on your machinery? I often run into this when I’m work­ing with cli­ents who use the terms to refer to ‘enabling devices’ – you know, those two or three-pos­i­tion switches that are found on robot teach­ing pendants and in oth­er applic­a­tions to give the oper­at­or a way to stop machinery, even if they have already been injured or killed by the equip­ment. Call­ing these devices a ‘Dead­man Switch’ or even a ‘Live-Man Switch’ as the three-pos­i­tion devices are some­times called, sends entirely the wrong mes­sage to the user as far as I’m con­cerned. The object­ive of our work as machinery safety engin­eers is to pre­vent injur­ies from hap­pen­ing in the first place. Using a device that is designed to determ­ine if the user is dead or uncon­scious means someone screwed up!

A little history

The term ‘dead­man’ comes from a device that was developed in the 1880’s by pion­eer­ing elec­tric­al engin­eer Frank Sprague. Sprague was work­ing on elec­tric trac­tion motor tech­no­logy, using these new machines to power street rail­ways (street­cars) and elec­tric elev­at­ors. The early DC motor con­trols used in both street­cars and elev­at­ors required an oper­at­or. The oper­at­or used a hand con­trol to move the street­car for­ward or back­ward along the track and to con­trol the speed of the car. In elev­at­ors, the oper­at­or used a sim­il­ar hand con­trol to move the elev­at­or car up or down the shaft, to con­trol the speed and to stop at the appro­pri­ate floor.

Westinghouse Streetcar Controller
1920’s era West­ing­house Street­car Con­trol­ler

If the oper­at­or was to doze off or fall uncon­scious, the street­car would simply con­tin­ue on its way until it hit some­thing or derailed, either being a poor option! Elev­at­ors would con­tin­ue until they hit the top or bot­tom of the shaft, again a bad idea. Sprague included a con­trol device in his designs that required the oper­at­or to keep his hand on the con­trol­ler handle and to main­tain pres­sure on the con­trol device in order for power to flow to the motor. This same idea was imple­men­ted in manu­al elev­at­or con­trol handles. These ideas were adop­ted by West­ing­house when they developed the street­car motor con­trol­lers that were used in thou­sands of street­cars run­ning between the 1890’s and the 1930’s.

When dies­el-elec­tric and full elec­tric loco­mot­ives were developed in the 1930’s, the concept of the ‘dead­man’ con­trol was adop­ted from street rail­ways. There is a per­sist­ent myth that these con­trols star­ted with steam loco­mot­ives. In fact, an earli­er ver­sion of this art­icle included that myth – now bus­ted!

A 'deadman' pedal in a locomotive.
A ‘dead­man’ ped­al in a dies­el-elec­tric rail­way loco­mot­ive

With the advent of elec­tric trams, trains, and sub­ways, con­cerns about pos­sib­il­it­ies like heart attacks and oth­er infirm­it­ies res­ult­ing in drivers los­ing con­trol of these machines caused these devices to be integ­rated into these new trans­port­a­tion sys­tems. To learn more about these applic­a­tions, see the Wiki­pe­dia art­icle Dead Man’s Switch.

It’s worth not­ing that the rail­ways now call these devices ‘Driver Safety Devices’ or DSD. See a mod­ern DSD at the Arrow­vale Elec­tron­ics web site.

Elev­at­ors moved from manu­al con­trol to auto­mat­ic con­trol, elim­in­at­ing the need for elev­at­or oper­at­ors and the need for ‘dead­man’ con­trols.

Robots Enter the Picture

Motoman robot pendant enabling device
Moto­man pendant with show­ing enabling device (red arrow)

In the 1980’s, indus­tri­al robots began to appear in the work­place. Acci­dents in these early days drove changes in the design of the con­trol pendants used to ‘teach’ these devices their tasks. Early pendants provided motion con­trol and an emer­gency stop device. Later, the motion con­trols were altered to become ‘hold-to-run’ devices that could jog the selec­ted robot axis at a pre-selec­ted slow-speed, one axis at a time. In the 90’s the ‘enabling device’ was added to the pendant. These two-pos­i­tion switches, still called ‘dead-man switches’, had to be held closed in order for the robot to move under con­trol of the axis hold-to-run con­trols. Acci­dents con­tin­ued to occur. In the mid 90’s the three-pos­i­tion enabling device, some­times called a ‘live-man-switch’, was intro­duced after stud­ies showed that some people would release their grip on the con­trol pendant when struck by the robot, while oth­ers would clench the hand hold­ing the pendant. The new switches are required to be held in the mid pos­i­tion to enable motion. The pic­ture at left shows the back of a mod­ern robot pendant. The black bar in the lower right is the enabling device, loc­ated so that your hand will nat­ur­ally hold the device in the cor­rect pos­i­tion when you hold the pendant in your left hand. Not so good if you are left-handed!

ABB IRB640 Robot Pendant
ABB IRB640 Robot Pendant

 

 

Euch­ner ZS Switches

In addi­tion to the pendant enabling devices, addi­tion­al enabling devices are required where more than one work­er is required inside the danger zone of the machine. These devices can be pur­chased sep­ar­ately and added to sys­tems as needed. Depend­ing on the applic­a­tion, you can get these devices with emer­gency stop but­tons and jog but­tons integ­rated into a single unit as shown in the pic­ture of the Euch­ner ZS switches.

Machinery Standards and Definitions

The enabling device is one of those pro­tect­ive meas­ures that can­not be read­ily clas­si­fied as a safe­guard­ing device because they do not pro­act­ively pre­vent injury. Instead, like an emer­gency stop, they may allow a work­er to avert or lim­it the harm that is already occur­ring. That places the enabling device into the ‘com­ple­ment­ary pro­tect­ive meas­ure’ cat­egory.

Let’s take a minute to look at a couple of import­ant defin­i­tions from the machinery stand­ards. At the moment, the best defin­i­tion for a com­ple­ment­ary pro­tect­ive meas­ure comes from the Cana­dian stand­ard, CSA Z432-04. Excerp­ted from CSA Z432-04, §6.2.3.5.3 Com­ple­ment­ary Pro­tect­ive Meas­ures:

Pro­tect­ive meas­ures that are neither inher­ently safe design meas­ures, nor safe­guard­ing (imple­ment­a­tion of guards and/or pro­tect­ive devices), nor inform­a­tion for use may have to be imple­men­ted as required by the inten­ded use and the reas­on­ably fore­see­able mis­use of the machine. Such meas­ures shall include, but not be lim­ited to,

a) emer­gency stop;

b) means of res­cue of trapped per­sons; and

c) means of energy isol­a­tion and dis­sip­a­tion.

Let’s also look at the form­al defin­i­tion of an ‘enabling device’ in the same stand­ard:

7.23.3 Enabling devices
7.23.3.1
An enabling device is an addi­tion­al manu­ally oper­ated 2- or 3-pos­i­tion con­trol device used in con­junc­tion with a start con­trol and which, when con­tinu­ously actu­ated in one pos­i­tion only, allows a machine to func­tion. In any oth­er pos­i­tion, motion is stopped or a start is pre­ven­ted.

7.23.3.2
Enabling devices shall have the fol­low­ing fea­tures:

a) They shall be con­nec­ted to a Cat­egory 0 or a Cat­egory 1 stop (see NFPA 79).

b) They shall be designed in accord­ance with ergo­nom­ic prin­ciples:

(i) pos­i­tion 1 is the off func­tion of the switch (actu­at­or is not oper­ated);

(ii) pos­i­tion 2 is the enabling func­tion (actu­at­or is oper­ated); and

(iii) pos­i­tion 3 (if used) is the off func­tion of the switch (actu­at­or is not oper­ated past its mid pos­i­tion).

c) Three-pos­i­tion enabling devices shall be designed to require manu­al oper­a­tion in order to reach pos­i­tion 3.

d) When return­ing from pos­i­tion 3 to pos­i­tion 2, the func­tion shall not be enabled.

e) An enabling device shall auto­mat­ic­ally return to its off func­tion when its actu­at­or is not manu­ally held in the enabling pos­i­tion.

Note: Tests have shown that human reac­tion to an emer­gency may be to release an object or to hold on tight­er, thus com­press­ing an enabling device. The ergo­nom­ic issues of sus­tained activ­a­tion should be con­sidered dur­ing design and install­a­tion of the enabling device.

 

OMRON A4EG Enabling Switches
OMRON A4EG Enabling Switches

Sim­il­ar defin­i­tions exist in the Inter­na­tion­al, European and US stand­ards, although they may not be quite as form­al­ized.

 

Most enabling devices on their own do noth­ing except PERMIT motion to take place, although the actu­al defin­i­tion of enabling device in CSA Z432-04 actu­ally per­mits the enabling device to cause motion. Absence of the enabling sig­nal pre­vents or stops motion. These devices are then used in con­junc­tion with hold-to-run con­trols on robots and machinery, and with throttle con­trols on trains, street cars, sub­ways and sim­il­ar equip­ment. Note that most stand­ards to not per­mit enabling devices to actu­ally cause motion. This is a unique situ­ation in the Cana­dian stand­ard.

So what’s the big deal?

Using the terms ‘dead-man’ or ‘live-man’ to describe these devices puts the wrong mes­sage out as far as I’m con­cerned. As safety engin­eers and OHS prac­ti­tion­ers, we care about keep­ing work­ers out of danger. This is neither check­ing to see if we have either a ‘dead man’ or a ‘live man’, but rather ensur­ing that the per­son in con­trol of the equip­ment is ‘in con­trol’.  Using a phrase like ‘enabling device’ clearly says what the device does.

In my opin­ion, and  sup­por­ted by the cur­rent Inter­na­tion­al and Cana­dian Stand­ards, these terms must be aban­doned in favour of ‘enabling device’ and the qual­i­fi­ers ‘2-pos­i­tion enabling device’ and ‘3-pos­i­tion enabling device’. These terms are also used in many of the cur­rent machinery safety stand­ards, so using them cor­rectly improves clar­ity in writ­ing and speak­ing. Clar­ity in com­mu­nic­a­tion in safety is too import­ant for prac­ti­tion­ers to per­mit the ongo­ing use of terms that con­vey the wrong mes­sage and do not pro­mote clar­ity of mean­ing. Since clar­ity is often lack­ing when it comes to safety, any­thing we can do to improve our com­mu­nic­a­tions should be high on our pri­or­ity list!

Ed. note: This post was updated on 17-Aug-17. The myth of dead­man con­trols on steam loco­mot­ives was removed and replaced by his­tor­ic­ally veri­fi­able inform­a­tion about the ori­gins of this con­trol.